The present invention applies to the production of optical elements used in lenses of the microscopic or conoscopic type and to the solving of a drawback inherent in current production methods, observed mainly when the angular field or the numerical aperture of these lenses becomes great.
The optical solutions adopted in the production of lenses of the “microscope” type, and in particular when the numerical aperture is large, use, normally and for lenses close to the object to be observed or measured, convergent lenses of the meniscus, plano-convex and more rarely biconvex type, which may, with regard to their convex part, be oriented towards to the image plane as far as a hemisphere. Various convergent lenses are shown on the first line of FIG. 1, while various lenses of the divergent type are shown on the second line of this FIG. 1.
This type of convergent lens is also used in a similar fashion in other types of equipment, in particular flux metrology or conoscopy.
Such a convergent lens may be formed as follows:                a first surface, oriented towards the object, spherical or aspherical, concave, planar or convex;        a transparent material with a refractive index >1. As a general rule the glasses used in optics have a refractive index of between 1.4 and 1.95;        a second convex, spherical or aspherical surface, which may range up to a form close to a hemisphere.        
The axes of revolution of the first and second surfaces are merged with the optical axis of the lens. The geometrical properties (radii of curvature) of these surfaces are such that the resultant lens is convergent (with a positive focal length).
The lenses that are most used in the context of optical assemblies with a high numerical aperture are general of the “convergent meniscus” type (“positive meniscus”).
So-Called “Total Internal Reflection” Phenomenon
It is well known that a light ray propagating in a medium with a refractive index n1 may, in a condition of incidence at an angle less than the critical theta angle (θc), be totally reflected where it encounters a medium with a lower refractive index n2.
The critical angle is given by: θc=arcsin(n2/n1).
In the context of a microscopic or conoscopic lens used for the applications mentioned above, the incident light emitted by the sample may perfectly, because of this phenomenon, be returned towards the sample, thus causing parasitic reflections that may compromise the observation thereof or the precision of the measurement.
For lasses with a very high index and in the case of a transition from the glass to air, the critical angle is very small (around 33° with glass of the S-LAH65 type with an index of 1.8 at 546 nm) as can be seen on the numerical simulation in FIG. 2. This simulation is given by way of example, the same type of phenomenon being able to be observed whatever the material used (here the glass of make Ohara with the reference S-LAH65).
This phenomenon possibly has important consequences for the properties of convergent lenses.
If the characteristics of the lens and the material used so permit, it is possible to observe this phenomenon experimentally, by pointing a laser in the directions and positions indicated in FIG. 4 for example: the first ray (solid line) passes normally through the system with a transmission coefficient 98.4%, the second (broken lines) undergoes two total reflections before returning towards the source with a reflection coefficient of 97.7%.
It is possible to simulate the behaviour of a lens of this type with ad hoc software tools, such as for example the Code V, ZEMAX or other programs. The simulation in FIG. 5 carried out with the ZEMAX software also shows the response of the component to an extended source placed in front of it. It is found that the region to which the total internal reflection phenomenon relates is annular in form.
The result of simulation of the reflected intensity in the plane of the object shows that, for an incoming flux of 1 W/cm2, the reflected intensity may attain 2.4 W/cm2.
Naturally these figures depend on the exact geometry of the meniscus in question.
It is on the other hand clear, in the light of the retroreflected energy levels (+240%), that this behaviour poses a metrological problem in the configuration of the system or of the pair consisting of sample to be measured and measuring system is such that the measurement is contaminated.